This invention relates to a dry toner and a dry developer used in fields of electrophotography, electrostatic recording, etc., and a process for forming images. More particularly, the present invention relates to a negative charge dry toner and a dry developer effectively used in a high-speed continuous paper (or serial) printer wherein a peripheral speed of a photoreceptor is 25 cm/sec or more, particularly 50 cm/sec or more, a high-speed cut sheet printer wherein a peripheral speed of a photoreceptor is 25 cm/sec or more and a printing speed of 60 sheets/min or more, a printer including a photoreceptor made of an organic photoconductive substance, and the like, and a process for forming images using such materials.
An electrophotographic process generally comprises a charging step for uniformly providing static charge on a photoreceptor using a photoconductive substance, an exposing step for forming a static latent image by irradiating a light, a developing step for attaching a toner to latent image portions, a transferring step for transferring to a toner image support, a fixing step for fixing the toner image to the image support with heat, pressure, flash light, or the like, a cleaning step for removing excess toner remaining on the photoreceptor, and a discharging step for returning to an original state as disclosed in U.S. Pat. No. 2,297,691 and British Patent Nos. 1,165,406 and 1,165,405. These steps are repeated to give a plurality of printed matters.
As toners for electrostatic image development used in the field of electrophotography, there have been proposed toners using polystyrene resins (Japanese Patent Examined Publication No. 44-16118), toners using styrene-acrylic resins such as toners using styrene-butyl methacrylate copolymer resin (Japanese Patent Examined Publication No. 56-11143), toners using bisphenol type epoxy resins obtained by reacting bisphenol and epichlorohydrin (Japanese Patent Unexamined Publication No. 57-96354), toners using polyester resins obtained by reacting a glycol having a bisphenol skeleton with a polybasic acid (Japanese Patent Examined Publication No. 52-25420), and the like. Among these others, the styrene-acrylic resins can widely been controlled to give proper resin properties such as molecular weights, glass transition points, molten viscosities, etc. and are extremely advantageous in designing toners, so that they have been used in large part as toners.
In order to provide desirable negative charge to these toners, there have been proposed to add substances for providing the negative charge to the resins to be bonded, for example, metal complexes of azo compounds (Japanese Patent Unexamined Publication Nos. 57-141452, 58-111049, 58-208750, etc.), metal complexes of oxycarboxylic acids (Japanese Patent Unexamined Publication Nos. 53-127726, 57-104940, 61-69073, etc.), halogenated paraffins (Japanese Patent Unexamined Publication Nos. 48-97542, 50-68140, etc.). It is also possible to prepare toner particles, followed by mixing a silica powder with the toner particles to adhere the silica powder to surface layers of toner particles.
These toners can be applied to appliances applying an electrophotographic method such as printers, copying machines, facsimiles. Particularly, in recent years, printers are increasingly used as terminals of computers for treating various information with high speed.
The printers can be divided into two types depending on kinds of paper used, i.e. a cut sheet printer wherein paper cut into predetermined size such as A4, B4, letter and legal sizes is used as a toner image support, and a continuous paper printer wherein continuous paper is used as a toner image support. The cut sheet printer is widely used for its advantages in that printing can be made on both front and rear sides of sheet of paper, high density printing is possible, handling is easy, and the like.
As photoconductive substances used in the above-mentioned appliances, inorganic substances such as amorphous selenium, zinc oxide, titanium oxide, cadmium sulfide, etc. heretofore used are replaced markedly by organic photoconductive substances having no problem in toxicity, excellent in transparency, flexibility and light weight, and being able to be produced inexpensively. A photoconductive member containing a photoconductive substance comprises an electroconductive layer and a photosensitive layer formed thereon. In organic photoconductive members, a function separation type comprising a charge generating layer and a charge transport layer has been evaluated recently due to excellency in sensitivity.
Generally speaking, as organic compounds generating electric charge and contained in the charge generating layer, there are known pigments such as azoxybenzene series, disazo series, trisazo series, benzimidazole series, polycyclic quinoline series, indigoid series, quinacridone series, phthalocyanine series, perylene series, methine series, etc. (Japanese Patent Unexamined Publication Nos. 47-37543, 47-37544, 47-18543, 47-18544, 48-43942, 48-70538, 49-1231, 49-105536, 50-75214 and 50-92738). As organic compounds for transporting electric charge and contained in the charge transport layer, there are known pyrazoline derivatives [Journal of Photographic Science and Engineering, vol. 21, (2), p. 73 (1977)], oxazole derivatives (Japanese Patent Unexamined Publication Nos. 55-35319, 58-87557, and 58-182640), hydrazone derivatives (Japanese Patent Unexamined Publication Nos. 54-59143, 54-150128 and 55-46760), enamine derivatives (Journal of Imaging Science, vol. 29 (1), p. 7 1985), etc.
Toners heretofore used in such electrophotographic members containing an organic photoconductive layer have no problem in initial printing, but when several thousands of sheets are printed, there are readily generated printing obstacles such as lowering in printed letter density, unevenness of printed letter density, and unable to obtain visible images.
Further, the toners heretofore used generally have no problem in charging properties, but cause various troubles after repeated uses such as flying (or scattering) of toners due to lowering in charging properties, resulting in generating contamination of inside and outside of the appliances and contamination of back portions of printed letters (hereinafter referred to as "fogging"), or lowering in printed letter density due to too high charging properties, resulting in difficulty in reading. These troubles are particularly undesirable in printers for printing important papers such as insurances, accounts, resident cards, articles, etc., due to their special uses. These problems seem to occur often in high-speed printers wherein impact strength, frictional force and centrifugal force for toners are great, and peripheral speed of organic photoconductor (OPC) is 250 mm/sec or more.
Further, in the case of the cut sheet printer, there are some problems in comparison with the continuous paper printer. For example, in the case of transferring a toner image from a photoreceptor to an image support such as paper, counter polar electric charge with respect to toner is applied from the back side of the image support depending on transferring devices. In such a case, when the electric charge is larger, the transferring effect of toner is enhanced to obtain high quality images, but the probability of generating so-call "Jam", which is a phenomenon making the image support adhere to the transferring device, is enhanced. On the other hand, when the electric charge from the transferring device is made smaller, no jam is generated, but the transferring amount of toner is small to produce poor images. Further, when the back side is printed after printing front side and fixing step, there readily takes place disturbance of images accompanying water content change in the image support and generation of fine wrinkles.